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Old 05-21-2006, 11:04 AM
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Chevy small-block engine vibration

I have a beat-up 1-ton chevy from '77 with a 207 block 350.

At highway speed it vibrates like it has a bent driveshaft, but is an engine speed vibration (does it at the same rpm in every gear).

The bottem end was worn enough to hear the crank knocking, so the main and rod bearings were changed so that it will hold together for a few more months until a replacement can be built. Now the clearances are tight enough to not be audible (didn't measure the resulting clearance, since worn-out is worn-out), but the vibration remains.

What types of problem give an rpm specific vibration? It doesn't do it at low speed, and I can drive through it (if it is in the range of 3000 to 3500 rpm, I can run above this speed for reduced vibration).

The harmonic balancer doesn't look like like the ring is slipping.

Any ideas? -I'd rather have a few ideas before I start randomly swapping components trying to find the culprit.
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Old 05-21-2006, 01:16 PM
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The crank shaft could have warped slightly causing this problem. Just a worn out engine of this sorts will do.

The dampener my be slipping more then thought or your flywheel/flex-plate could be wearing out, have a crack.

Have you checked the engine mounts. If one of them is gone it can cause the engine to vibrate much more then it used to

The trans could being having some problem in the shafts causing the vibration. The transmission mount could have broke

More then likely though if the engine was knocking and the only thing you did since was replace the bearings... your problem is there...
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Old 05-21-2006, 01:32 PM
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re:chevy small-block engine vibration

had a 350 in a 78 jimmy once that did about the same idled fine but around 2500 rpm vibrated and it turnout to be the flex plate was warped,tomuch off roading
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